1. Field of the Invention
The present invention relates to oxidative hair dyeing processes, and particularly to a novel and unexpected method for conforming the oxidative dyeing rate of virgin hair to that of non-virgin hair, resulting in substantial improvements to the efficiency of the process.
2. Background of the Prior Art
Oxidative hair dyeing processes are generally distinguished from other hair dyeing processes, by virtue of an essentially in-fiber synthesis of color species generated from a three component reactive composition. The three reactive components contained in an oxidative hair dyeing composition are as follows:
(1) The primary dye intermediates, PA0 (2) The color couplers or modifiers, these components 1 and 2 being commonly referred to as dye precursors, and PA0 (3) The oxidizers.
Hair dyed using this process retains its color until the color is chemically removed by bleaching or other processes. The process is thus deemed to be a permanent dyeing process.
Generally, the process is conducted at alkaline pH 9 to 10, requires 20 to 40 minutes, and usually employs hydrogen peroxide as the oxidizer.
Oxidative damage to hair, after a single dye application, from such compositions, may be small. However, when these dye applications are repeated, or when combined with other cosmetic treatments such as permanent waving, relaxing, or extensive bleaching, the hair damage is considerably more severe. Such damage is normally experienced by the consumer in the form of coarse feel to the hair and tendency of the hair to break-off.
Consumers generally have their hair dyed periodically, typically once a month, sometimes as often as twice a month. Between each hair dyeing session, new hair grows out from the scalp. Thus, it is seen that typically the hair to be dyed comprises virgin hair (proximate the scalp) and previously dyed hair (proximate the virgin hair). Moreover, the previously dyed hair may comprise hair that has undergone any number of discrete permanent dyeing treatments, depending on its length and frequency of dyeing treatments. Although hair dyeing treatments are those most often involved, the nonvirgin hair may have been also subjected to any of the other chemical treatments, such as permanent waving, enumerated above.
It is well known that when hair is treated with peroxide or other oxidizers, an essential amino-acid called cystine, which is found in hair, undergoes oxidation to cysteic acid. This chemical reaction leads to a weakening of the hair structure, making the hair dryer and prone to breakage. Accordingly, there is reason for consumers to believe that exposure to hydrogen peroxide during the oxidative dyeing of hair is a primary causative factor responsible for the damage to hair.
The hair coloring industry has attempted to satisfy this consumer concern by reformulating oxidative hair dyes with metal ion catalyst that hastens the peroxide oxidation reaction of the dye precursors and thus reduce the amount of time hair is exposed to the damaging effects of hydrogen peroxide or other oxidizers.
However, these catalytic oxidative dyeing processes have met with little or no commercial success for several reasons. First, it is well known that molecular diffusion of dye materials into nonvirgin hair or cosmetically treated hair, occurs faster than diffusion into virgin hair. Accordingly, the presence of catalyst, which hasten the oxidative dyeing process, itself serves to further disproportionate the uneven rate of dyeing of the new hair growth as compared to that of previously treated hair. Secondly, the nonvirgin or previously treated hair has a greater affinity for the metal catalyst ion than does virgin or new hair. Thus, there is a greater amount of the metal ion catalyst deposited on the nonvirgin hair, where the molecular diffusion of the dye materials also takes place more rapidly. This serves only to aggravate the uneven or disproportionate dyeing of the two types of hair.
These disproportionate dyeing rates have in the past necessitated the use of metal ion catalyst concentrations that are high enough to provide a sufficiently deep shade of color in the virgin hair so that the faster accumulation of color in the nonvirgin hair is less perceivable. Thus, an excessive accumulation of metal ions necessarily takes place. Such high metal ion concentrations on the hair, when in contact with alkaline peroxide, can cause exothermic reactions to take place which lead to severe discomfort and damage. Higher metal ion concentrations also inherently create a coarse hair feel. Therefore, despite the shorter treatment time made possible from the metal ion catalyst, no noticeable decrease in hair damage is apparent to the consumer.
Patents issued which are exemplary of these prior art drawbacks are set forth below. German Patent DE 2,028,818 to the Gillette Company discloses an oxidative dyeing process for hair wherein the hair is pretreated with a solution of metal salt at concentrations of from about 0.01 to 0.15 Molar for 5 minutes. The hair is then rinsed with water and exposed to a solution containing dye precursors and hydrogen peroxide at alkaline pH of about 9.5 for 5 minutes. This dyeing time is significantly shorter than the time for conventional oxidative dyeing processes. However, the required concentration of metal ions to sufficiently catalyze acceptable levels of dyeing in virgin hair was so high that tensile strength measurements of the hair have indicated no noticeable decrease in hair damage as compared to the conventional processes, notwithstanding the acceleration of the oxidative reaction. The attendant risk of perceivable uneven dyeing, absent severe metal build-up and damage, leaves this process of little commercial value.
International patent W088/01162 to Goldwell discloses a procedure for low pH dyeing wherein metal salt catalysts are added directly to a cream or gel-like oxidative dye mixture, rather than applying the catalyst separately to hair as a pretreatment. Because low metal concentrations were employed, the Goldwell process is suitable solely for treating nonvirgin (or previously cosmetically treated) hair to temporarily tint or tone the hair. The Goldwell process cannot completely dye virgin, i.e., chemically unaltered, hair to obtain a permanent and intense color.
A method for oxidatively dyeing hair in a rapid but permanent fashion, which changes the chemistry of this reaction on hair, and particularly which could conform the rate of oxidatively dyeing virgin hair to approximate that of the faster nonvirgin hair rate, while substantially reducing metal accumulation, would represent an unexpected advancement in the art and would satisfy a long felt need in the industry.